ARTICLE https://doi.org/10.1038/s41467-020-17262-w OPEN First look by the Yutu-2 rover at the deep subsurface structure at the lunar farside ✉ Jialong Lai 1,2,YiXu 1 , Roberto Bugiolacchi 1,3, Xu Meng1,4, Long Xiao1,5, Minggang Xie 1,6, Bin Liu7, Kaichang Di7, Xiaoping Zhang1, Bin Zhou 8,9, Shaoxiang Shen8,9 & Luyuan Xu 1 The unequal distribution of volcanic products between the Earth-facing lunar side and the farside is the result of a complex thermal history. To help unravel the dichotomy, for the first 1234567890():,; time a lunar landing mission (Chang’e-4, CE-4) has targeted the Moon’s farside landing on the floor of Von Kármán crater (VK) inside the South Pole-Aitken (SPA). We present the first deep subsurface stratigraphic structure based on data collected by the ground-penetrating radar (GPR) onboard the Yutu-2 rover during the initial nine months exploration phase. The radargram reveals several strata interfaces beneath the surveying path: buried ejecta is overlaid by at least four layers of distinct lava flows that probably occurred during the Imbrium Epoch, with thicknesses ranging from 12 m up to about 100 m, providing direct evidence of multiple lava-infilling events that occurred within the VK crater. The average loss tangent of mare basalts is estimated at 0.0040-0.0061. 1 State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China. 2 School of Science, Jiangxi University of Science and Technology, Ganzhou, China. 3 University College London, Earth Sciences, London, UK. 4 School of Civil Engineering, Guangzhou University, Guangzhou, China. 5 Planetary Science Institute, School of Earth Sciences, China University of Geosciences, Wuhan, China. 6 College of Science, Guilin University of Technology, Guilin, China. 7 State Key Laboratory of Remote Sensing Science, Aerospace Information Research Institute, Chinese Academy of Science, Beijing, China. 8 Key Laboratory of Electromagnetic Radiation and Detection Techonology, Chinese Academy of Sceience, Beijing, China. 9 Aerospace ✉ Information Research Institute, Chinese Academy of Science, Beijing, China. email: [email protected] NATURE COMMUNICATIONS | (2020) 11:3426 | https://doi.org/10.1038/s41467-020-17262-w | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17262-w nraveling the shallow subsurface structure of the lunar lasted about 200–600 Ma21,31, with the youngest flows estimated Umare offers the key to a better understanding of the local between 3.15 and 3.6 Ga19,21. However, currently, no direct evi- history of basaltic volcanism, an important process cou- dence of the volcanic history of VK crater indicates whether the pled to the Moon’s thermal evolution1. The thickness and surface mare deposits were formed by one episode of basaltic volcanism area of basalt layers can be used to constrain lava eruption based on the uniform reflectance spectral characteristics or volumes. A range of remote-sensing data including the study of multiple lava-infilling events19. LPR can provide first-hand data impact craters morphology2,3, the analysis of high-resolution to disclose the subsurface stratigraphy and constraint the thermal gravity data4, and the reflectance spectra of crater ejecta depos- history. its5–8 have contributed to developing the current model of lunar VK’s neighboring region is geologically highly complex: the evolution. map I-104731 and the inset32 (Supplementary Fig. 1) show a Ground-penetrating radars on the lunar surface and radar superposition of impact morphologies spanning from the pre- sounders onboard orbiting spacecraft have helped to investigate Nectarian to the Copernican epochs. The neighboring impacts the physical properties of the subsurface materials and their produced ejecta materials that punctuated the infill and post-infill possible stratigraphy. The Apollo Lunar Sounder Experiment, phase of the VK crater. The time sequence of these craters is part of the Apollo 17 mission, was the first instrument to detect relevant to the interpretation of the stratigraphy at the CE-4 deep subsurface reflectors corresponding to the interface between exploration path, which is analyzed in Supplementary Note 1. mare and bedrock9 at average apparent depths of 1–1.6 km in Mare Serenitatis, Mare Crisium, and Oceanus Procellarum9–11. The apparent depth is defined as the propagation depth of a radar Lunar-penetrating radar results. The penetrating depth of LPR signal with the speed of light in the vacuum. Later, the Lunar CH-1 can reach up to ~330 m (Supplementary Note 2 and Sup- Radar Sounder onboard the Kaguya spacecraft (SELENE) plementary Fig. 2), although the top section of the radar signals observed relatively shallow reflectors interpreted as subsurface becomes saturated due to the strong coupling effects from the boundaries between distinct basaltic rock layers in the nearside electromagnetic interaction with the metal in the rover. However, maria at apparent depths in the range of hundreds of meters12–14. channel two (CH-2) of the LPR data, the center frequency of Compared with the spaceborne radar experiment, the lunar- which is 500 MHz and can penetrate up to ~35 m17, can be penetrating radar (LPR) onboard Chang’e-3 (CE-3) and Chang’e- employed to complement the profile of the close-to-surface sec- 4 (CE-4) rover have a much higher range resolution (1–2 m in the tion17 (Fig.1c). Here we focus on the LPR data analysis between mare basalt for the 60 MHz channel), thus offering a unique 52 and 328 m. opportunity to survey in greater detail the shallow subsurface of The prominent and continuous subsurface reflectors A–Eat both the lunar nearside and farside15–17. CE-4 landed in the depths of (A) 51.8 ± 1.1 m, (B) 63.2 ± 1.2 m, (C) 96.2 ± 3.2 m, (D) South Pole-Aitken (SPA) Basin, the largest known impact 130.2 ± 3.7 m, and (E) 225.8 ± 5.5 m can be observed both in the structure on the Moon and a key region ideally suited to address processed radar image and aggregated data traces displayed in several outstanding geological questions as the impact might have terms of signal strength (dB, yellow line) (Fig. 1a). The horizontal even penetrated the entire lunar crust18,19. The radargrams pro- reflectors appear relatively constant running parallel to the duced from the data acquired by the CE-4 instrument reveal the surface (see Fig. 1), except for the horizontal reflectors D that basalt layer thickness of each lava eruption and the time sequence shows a gradual rise of 7.1 m in the right end. This is probably of surface modification events that occurred in the Von Kármán due to the change in subsurface topography, e.g., crater at depth (VK) crater (Supplementary Fig. 1). In a broader context, this of 130 m (see simulation results in Supplementary Fig. 4). From new information adds to our limited understanding of the around waypoint 42, the reflector D becomes flat, because the igneous history of the SPA Basin, which is thought to have been rover conducted a local exploration mission to collect other significantly shorter and less extensive than its equivalent on the scientific data at the end of the ninth month exploration with nearside1,20. The reason for the asymmetric distribution between consequent little variation of the subsurface topography. None- the lunar sides is understood to relate either to differences in theless, this localized and repeated sampling phase helps to crustal thickness, to the abundance of radioactive elements, or to constrain the consistency and reliability of the data gathering the geological consequences of the large SPA-forming impact process. itself21–27. The materials between the most prominent horizontals are In this work, we report the LPR results for the first 9 months rather uniform and strong radar echo are rare (e.g., A–B, B–C, derived from channel one (CH-1, 60 MHz) data and test our C–D in Fig.1b); however, a couple of subtle features stand out at interpretations using LPR simulation. A stratigraphic model of the the bottom part region of the radargram using image enhance- surveying area (landing coordinates based on Lunar Reconnais- ment technique. Some relatively short lines appear in the D–E sance Orbiter terrain data: 177.5885°E, 45.4561°S, −5927 m28,29) strata and more continuous ones occur below reflector E, which was generated from the extracted reflectors profile, which suggests are interpreted as ejecta at a different scale. As the thickness of possible lava flows sources and a potentially complex buried stratum D–E is about 100 m, it is possible that it was formed by topography. The local geological history of the CE-4 landing site is multiple lava eruption events interposed by small-scale ejecta inferred based on the revealed stratigraphy. deposits or thin regolith that was formed in the lull period: this geologically complex admix may reflect in the scattered features in the radar results as evident in Fig.1b. Alternatively, the large- Results scale ejecta layers at depths of over 200 m may produce relatively Geologic settings. VK crater (171 km) lies within the SPA basin, continuous signal discontinuities, but with more pronounced an impact crater about 2500 km in diameter. The thermal history fluctuations than a well-defined interface. of the crater and its neighborhood thus should be interpreted within an atypical geological context19–25. During the Late Heavy Bombardment (LHB30) period, several giant impacts including Simulation results. To test our geological interpretation of Imbrium on the nearside and VK’s northern neighbor, crater the radar data, several subsurface models were designed for LPR Leibnitz (245 km in diameter) were produced. Post LHB, the simulation with various sets of loss tangent and permittivity region underwent a relatively prolonged phase of lava infill, which values.